1. Field of the Invention
The invention relates to a drive control apparatus and method for controlling an alternating current motor, hereinafter referred to as an “AC motor”, subjected to rotational drive by applying a rectangular wave voltage thereto.
2. Description of the Related Art
For outputting a predetermined command torque from a motor, it is widely known to use a technique of controlling voltage applied to the motor. For example, Japanese Patent Laid-Open Publication Nos. 2001-28892 and 2000-50689 respectively disclose a drive control apparatus which calculates such a voltage phase so as to eliminate deviation of an output torque value from a command torque value by feeding back an output torque value of a motor. Japanese Patent Laid-Open Publication No. 2001-28892 obtains an output torque value based on respective phase currents detected by a detector, a command voltage value, and an angular speed. On the other hand, Japanese Patent Laid-Open Publication No. 2000-50689 obtains an output torque value using a torque sensor.
Furthermore, Japanese Patent Laid-Open Publication No. Hei 10-14273 discloses a control apparatus of a motor for controlling an inverter phase angle so that a command torque value may be outputted. This control apparatus obtains a torque compensation angle based on a deviation of an output torque value from a command torque value, and an angle (a voltage vector angle) to the flux axial direction of a voltage vector using a motor model with a motor in a simulated state and the command torque value. An inverter phase is calculated based on the obtained torque compensation angle and the voltage vector angle.
In a detection process of an output torque value, usually, there is provided a low pass filter for filtering out noise. Especially, for calculating the output torque value based on detected values of respective phase currents, noise filtering by the low pass filter is essential because the detected current value includes a harmonic component. However, use of the low pass filter causes a time lag in the obtained output torque value, thus degrading torque response. The drive control apparatuses disclosed in Japanese Patent Laid-Open Publication Nos. 2001-28892 and 2000-50689, which only feed back the output torque, have a disadvantage that the torque response of a motor is low.
A control apparatus disclosed in the Japanese Patent Laid-Open Publication No. Hei 10-14273 makes combined use of feedback control and feed-forward control, which is capable of suppressing an effect of time lag in an output torque. The control apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei 10-14273 calculates a voltage vector angle from a torque command. That is, it may be said that the voltage vector is a parameter with state quantity of a motor not considered. Therefore, there is a problem that a command torque value that changes in a stepped manner will also cause a voltage vector angle to change in a stepped manner, thus causing the output torque of the motor to have an overshoot state. In other words, conventional drive control of a motor has a disadvantage that low response or overshoot causes a control error, especially a significant control error in a transient region.
In view of the aforementioned problems, it is an advantage of the present invention to provide a drive control apparatus and method capable of further suppressing a control error.